1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof and more particularly to an improvement on a groove structure used for resistive isolation between elements.
2. Description of the Prior Art
FIG. 9 is a partial cross-sectional view showing the structure of a conventional reverse-conducting gate turn-off thyristor (reverse-conducting GTO) 100 in the vicinity of an element isolation region. FIG. 9 corresponds to a cross section of a part 100a of the reverse-conducting GTO 100 taken along the line IX--IX of FIG. 10 which is a plan view thereof.
The reverse-conducting GTO 100 comprises a semiconductor body 110, which has a pnpn four-layer structure consisting of an n emitter layer 111 (n.sub.E), a p base layer 112 (p.sub.B), an n base layer 113 (n.sub.B) and p emitter layers 115 (p.sub.E). Although FIG. 9 shows a single n emitter layer 111, a large number of n emitter layers 111 are formed on the p base layer 112. Between the respective p emitter layers 115 is provided an n-type high impurity concentration region 116 (n.sup.+).
The semiconductor body 110 incorporates a GTO and a diode in reverse parallel. A region GR is a GTO region, which is connected to an external circuit through a cathode layer 122 (K), gate electrode layers 123 (G) and an anode layer 124 (A). A region DR is a diode region, in which an electrode layer 121 is used as an anode of the diode and the electrode layer 124 functions also as a cathode of the diode. The electrode layer 121 is electrically connected to the cathode layer 122.
An element isolation region SR is provided between the GTO region GR and the diode region DR. In the element isolation region SR is formed an element isolation structure 130 having an isolating groove 131. The isolating groove 131 is formed by selectively etching the p base layer 112. Overetched portions 134 and 135 are formed at the foot of sidewalls 132 and 133 of the isolating groove 131. A p.sup.+ isolation layer 114 is formed selectively in the part of the n base layer 113 under the isolating groove 131.
A p region 112c which is the part of the p base layer 112 at the bottom of the isolating groove 131 has a function of resistively separating the p layer 112 into a part 112a in the GTO region GR and a part 112b in the diode region DR by sheet resistivity thereof. That is, since isolation resistance by the sheet resistivity of the p region 112c is inserted equivalently between the electrode layers 121 and 123, a leakage current between the gate and the cathode through the p region 112c can be reduced.
FIG. 11 is an enlarged view of the isolating groove 131. Under the same diffusion conditions and the like, the sheet resistivity of the p region 112c generally depends on the thickness of the p region 112c. The center of the bottom 136 of the isolating groove 131 is different from the overetched portions 134 and 135 in thickness of the p region 112c. The width W.sub.a of the respective overetched portions 134 and 135 is quite smaller than the overall width W of the isolating groove 131. Therefore the isolation resistance of the reverse-conducting GTO 100 by the region 112c is substantially determined by the thickness D.sub.a of the p region 112c in the center of the bottom 136 of the isolating groove 131. The smaller the thickness D.sub.a is, the higher isolation capability is.
When a forward voltage is applied to the GTO region GR, a depletion layer extends in the p layer 112. FIG. 12 shows typically the extension of the depletion layer in the vicinity of the overetched portion 134 enclosed by the broken line in FIG. 9. As the forward applied voltage increases, the extension quantity of the depletion layer 140 increases, so that the top 141 thereof approaches the overetched portion 134 to be exposed to the isolating groove 131. The influence of ions and the like adhering on the overetched portion 134 causes local electric field concentration on the top of the depletion layer 140, sometimes resulting in breakdown of the reverse-conducting GTO 100. For improving a forward breakdown voltage of the reverse-conducting GTO 100, the thickness D.sub.b (in FIG. 11) of the p region 112c at the deepest part of the overetched portions 134 and 135 should be large.
In the conventional reverse-conducting GTO 100, however, increase in the thickness D.sub.b inevitably results in increase in the thickness D.sub.a of the center because of a D.sub.b &lt;D.sub.a relation. There is a trade-off relation between the isolation resistance and the forward breakdown voltage, and it is difficult to improve both of them at the same time. FIG. 13 is a graph illustrating such a state, in which the forward breakdown voltage starts to drop when the isolation resistance exceeds about 70.OMEGA.. The broken line in FIG. 13 represents a theoretical value of the breakdown voltage of p.sub.B n.sub.B junction.